[Amps] Doorknob Caps Needed: Background

[Bill, W6WRT]

ORIGINAL MESSAGE:

On Thu, 28 May 2009 04:31:49 +0000, RICHARD SOLOMON <w1ksz at q.com>
wrote:

>There must be some way to determine the appropriate value 
<snip>

REPLY:

The value of the coupling cap is probably the least critical of any
capacitor in the entire amplifier. Its current and voltage rating must
be equal to the task, but the capacitance - within reason - is
relatively unimportant. 

A typical tube amplifier will have a plate load impedance in the range
of 1000 to 3000 ohms or so. The coupling cap should have a reactance
at least ten times less than the plate load impedance, so at six
meters, a value of anywhere from 100 pF to 10,000 pF or even larger
will do just fine.  

For this application, I like the Russian doorknob caps available on
eBay and elsewhere. They are built like a tank and have proven highly
reliable. Do not be tempted to use the relatively small high voltage
caps used as filters in CRT HV supplies. They do not have the current
carrying capacity. 

73, Bill W6WRT